Why does my booking page ask the guest to send me a hello on WhatsApp?

Because WhatsApp only lets a business write to somebody who has written to it first. The booking page gives the guest a button that sends you a short hello with their booking number in it, and their confirmation goes the moment that hello arrives. It waits up to fifteen minutes; if they never write, the confirmation is not sent, but the booking is still on your list either way. Your managers' group and your email get the booking straight away regardless. The Tables, tabs and the kitchen screen tab in the docs explains the confirmation.